Conventional machine tool and robotic apparatus use a numerical control apparatus to perform control such that the position of a drive shaft follows a command, but the position detected by a position sensor is different from the position of a part originally intended to be controlled. For example, a parameter acquired by a position sensor of a machine tool is a parameter of the angle of rotation of a motor or the position of a table or a spindle head mechanically connected to the motor. However, an implement end which is the part originally intended to be controlled is a workpiece fixed to the table or the tip of a tool attached to the spindle. Accordingly, the implement end which is the part originally intended to be controlled does not follow a command position perfectly even when it is controlled to follow the command position while feeding back the value acquired by the position sensor, whereby a desired machining result may not be obtained.
Moreover, for example, a machining point at which the workplace is in contact with the tool or the motor itself generates heat to become a heat source, and this heat is transmitted to the apparatus and causes minute deformation of the apparatus, whereby a desired machining result may not be obtained. A test operation is thus performed by attaching a measuring ins eminent to the implement end at the time of adjustment and maintenance of the apparatus, so that the numerical control apparatus addresses the above situation by performing correction processing that corrects a deviation between the detected position obtained from the position sensor on the basis of a measured result of the test operation and an actual position of the implement end, as well as corrects a shift in the position of the implement end caused by the deformation of the apparatus. In order to accurately perform the above correction processing, it is desirable that information from an external measuring instrument and information from the position sensor communicated within the numerical control apparatus are obtained in synchronization with each other timewise, where an example of such method is disclosed in Patent Literature 1.
A controller described in a first embodiment of Patent Literature 1 uses an unused analog signal input unit of a servo amplifier to acquire a signal via a communication unit connecting the servo amplifier and a numerical control apparatus, and thus acquires a signal obtained from a position sensor or a speed sensor of the servo motor in synchronization with a signal obtained from an external measuring instrument. Such a controller however has a problem that the number of external measuring instruments allowed to perform measurement at the same time is limited to the number of unused analog signal input units included in the servo amplifier. A controller described in a second embodiment of Patent Literature 1 solves this problem by providing an interface circuit unit that acquires a signal from an external measuring instrument. However, such a controller communicates a signal from the external measuring instrument to a control unit via a communication unit connecting a servo amplifier and a numerical control apparatus, where an attempt to acquire many signals at the same time such as when a plurality of external measuring instruments is used causes an increase in the communication load. This communication unit with the increased communication load is originally used by the control unit to communicate a command position that controls the driving of the servo motor to the servo amplifier, and by the servo amplifier to communicate a signal from a position sensor or speed sensor of the servo motor to the control unit in order to feed back the signal. The communication unit thus transmits and receives data in a short communication cycle in order to control the servo motor at a high speed.